Through the use of recently perfected intracellular pH microelectrodes, with an outer tip diameter less than or equal to 700 A, a slope greater than or equal to 55 mV/decade pH change, an impedance between 750 to 1,000 megs with a response time less than or equal to 100 microseconds, changes in directly determined intra- and extracellular pH will be continuously recorded. Tissues will be subjected to hypoxia, metabolic inhibitors, and selected pharmacological and physiological interventions. Tissue responses, as indicated by pH, transmembrane potentials, contractility and systolic-diastolic pressures, will be continuously recorded for subsequent analysis to establish the interrelationships between these recorded events. Another analysis will attempt to determine whether or not there is equilibrium across the cell membrane for hydrogen as predicted by the membrane potential; this analysis will be similar to that employed to correlate the changes in pH with potassium and sodium ions. Myocardium from dog (Purkinje), rabbit (papillary), and guinea pig (atria, strips or right ventricle and Langendorff perfused hearts) will be used to provide data on species variation and differential activities for comparison between normal response with changes induced by the above indicated agents and procedures. These data may provide a basis to explain some cellular and tissue activities as related to intracellular pH changes.